I certainly accept that a non rebounding hammer - in the lowered position - can be fired by a branch which lifts it almost to the half cock and then releases it or if dropped a swift tap on the hammer could also fire the gun. To walk around with a non rebounding hammer in the lowered position would be courting disaster. In the half cock position it should be as safe as a modern gun with the safety on - or even safer - as long as it isn't worn. Most people would not do walked up shooting over here with a cocked hammer gun, either fitted with rebounding of non rebounding as both can fire if the trigger is pressed or possibly dropped, if a little worn.
I have to say I do use modern guns if I am expecting to do walked up shooting. I treat the gun like a cocked hammer gun because that is what it is. On all my guns the safety locks the triggers - not the hammers - they are only held back on a piece of metal the thickness of a finger nail - and my life depends on that bit of metal staying in engagement. Would you feel comfortable dangling at the end of a rope from a bridge, 100 feet above a gorge, with just a sear and bent linking you to the bridge? and yet we do it all the time with a gun.
Obviously no one points a loaded gun intentionally at anyone else so this is the ultimate safety procedure - but how many people on a driven day remember to slip the safety back on as soon as they have fired the gun before bringing it back down through the line - not everyone I suspect.
As an instructor I have to make everyone aware of the risks and the procedures to reduce these risks to the absolute minimum. If you have a hammer gun it can be used safely - if you follow the procedures - OK it might not be the quickest gun to use in certain situations.
If you use a hammerless (better described as a concealed hammer gun) then I make people aware that they are carrying a fully cocked gun around at all times and they must take appropriate measures to ensure they handle it safely - so if a worst case scenario happens they don't shoot someone.
John